1. Field of the Invention
The present invention relates to a spread spectrum signal reception apparatus for receiving the code division multiplex signal.
2. Related Background Art
The present applicant proposed a spread spectrum signal reception apparatus which controls the amplifying ratio of an amplifier, in Japanese Patent Application No. 3-304005 (Japanese Patent Application Laid-Open No. 5-122192), by utilizing the peak of a correlator output as an automatic gain control (AGC) voltage.
The constitution of a reception apparatus explained in Japanese Patent Application Laid-Open No. 5-122192 is shown in FIGS. 4 and 5.
Next, an operation of the circuit shown in FIG. 4 will be explained.
A convolver output detected by a detector 41 is further detected of its peak value by a peak level detector 22.
An output of a detector 41 is input to a noise level detector 23 and the noise level is detected by this noise level detector 23. A noise level detector 23 interrupts only the regular peak portion of a convolver output and detects the peak of another portion.
Then, the difference between the peak level and the noise level is detected by a difference detector 24. This difference corresponds to the ratio of the convolver output signal to the noise. When this difference voltage becomes higher, the S/N ratio of a convolver output is improved. Accordingly, gains of two variable amplifiers 13 and 19 are controlled to become the appropriate value respectively from a control voltage generator 25 so that this difference voltage becomes a certain predetermined value V.sub.0. Accordingly, the S/N ratio of a convolver output can be held at the best condition irrespective of variation of the reception wave level, with maintaining the output voltage of a difference detector 24 at the previously set value V.sub.0. It should be noted that a first local oscillator may be controlled instead of an amplifier 13.
Next, an operation of the circuit shown in FIG. 5 will be explained.
The correlation output of a convolver 14 has the known output interval t.sub.1 given with the transference rate of information to be transferred. Also, the width t.sub.2 of the correlation output is already known. The correlation output is sent to a detector 41 through a high frequency switch 33 and further sent to a code synchronization unit.
In case where the code is synchronized by detecting the peak of the most large correlation output depending on an envelope detection, if a correlation output of the delay wave component depended on the multi-pass is observed, it becomes difficult to synchronize the code because the peak interval is deviated due to the delay wave.
Accordingly, in a circuit shown in FIG. 5, a high frequency switch 33 is switched responding to the interval of a correlation output depended on a desired wave component to eliminate the correlation output component due to the delay wave.
When the switch signal of a switch signal generator 34 is on-state, a high frequency switch 33 becomes through-state and the correlation output is sent to a detector 41. That is, when the switch signal is off-state, a high frequency switch 33 acts as an attenuator to attenuate the correlation output, and when the switch signal is on-state, a high frequency switch 33 becomes through-state to conduct the correlation output to the detector 41 side.
Accordingly, the correlation output eliminated delay wave component can be obtained by fitting the cycle of the switch from a generator 34 to the interval t.sub.1 of the above correlation output, and fitting the on-timing of the switch signal to the timing of the desired wave, further by setting the on-pulse width t.sub.3 of the switch signal narrower than the width t.sub.2 of the above correlation output (t.sub.3 &lt;t.sub.2).
The switch signal from a generator 34 is maintained at the on-state until the initial correlation output is obtained after the power is turned on. Due to this fact, the initial correlation output is sent to a detector 41 and the detection output is input to a timer circuit 35. According to this, a timer circuit 35 is operated and the timer output is switched from an off-state to an on-state with the timing (t.sub.m) responded to the interval t.sub.1 of the above correlation output to turn on the switch signal of a switch signal generator 34. Then, from the second correlation output, the switch signal is ON/OFF controlled in response to the interval t.sub.1 of the correlation output by the own operation of a timer circuit 35. That is, the on-pulse width t.sub.3 and the off-pulse width (t.sub.1 -t.sub.3) are set together with the initial off-time t.sub.m in a time circuit 35.
However, in a burst synchronization system, if the AGC is utilized, the ratio for an input level of the correlator output for extracting the synchronization signal is greatly changed between the burst synchronization period and the multiplex demodulation period subsequently followed, and the AGC output level does not become a constant level. Therefore, an operation point is greatly changed in a later stage, so that the circuit design becomes difficult.
During the multiplex demodulation period, it is difficult to realize the accurate AGC by only the synchronization signal in a correlator output because of the influence of interference due to the mutual correlator of channels other than synchronization signal channel.